It is well-known that the most efficient conversion of solar energy to electrical energy with the least thermalization loss in semiconductor materials is accomplished by matching the photon energy of the incident solar radiation to the amount of energy needed to excite electrons in the semiconductor material to transcend the bandgap from the valence band to the conduction band. However, since solar radiation usually comprises a wide range of wavelengths, use of only one semiconductor material with one bandgap to absorb such radiant energy and convert it to electrical energy will result in large inefficiencies and energy losses to unwanted heat.
Ideally, there would be a semiconductor material with a bandgap to match the photon energy for every wavelength in the radiation. That kind of device is impractical, if not impossible, but persons skilled in the art are building monolithic stacks of different semiconductor materials into devices commonly called tandem converters and/or monolithic, multi-bandgap or multi-bandgap converters, to get two, three, four, or more discrete bandgaps spread across the solar spectrum to match more closely to several different wavelengths of radiation and, thereby, achieve more efficient conversion of radiant energy to electrical energy. Essentially, the radiation is directed first into a high bandgap semiconductor material, which absorbs the shorter wavelength, higher energy portions of the incident radiation and which is substantially transparent to longer wavelength, lower energy portions of the incident radiation. Therefore, the higher energy portions of the radiant energy are converted to electric energy by the higher bandgap semiconductor materials without excessive thermalization and loss of energy in the form of heat, while the longer wavelength, lower energy portions of the radiation are transmitted to one or more subsequent semiconductor materials with smaller bandgaps for further selective absorption and conversion of remaining radiation to electrical energy.
Semiconductor compounds and alloys with bandgaps in the various desired energy ranges are known, but that knowledge alone does not solve the problem of making an efficient and useful energy conversion device. Defects in crystalline semiconductor materials, such as impurities, dislocations, and fractures provide unwanted recombination sites for photo-generated electron-hole pairs, resulting in decreased energy conversion efficiency. Therefore, high-performance, photovoltaic conversion cells comprising semiconductor materials with the desired bandgaps, often require high quality, epitaxially grown crystals with few, if any, defects. Growing the various structural layers of semiconductor materials required for a multi-bandgap, tandem, photovoltaic (PV) conversion device in a monolithic form is the most elegant, and possibly the most cost-effective approach.
Epitaxial crystal growth of the various compound or alloy semiconductor layers with desired bandgaps is most successful, when all of the materials are lattice-matched (LM), so that semiconductor materials with larger crystal lattice constants are not interfaced with other materials that have smaller lattice constants or vice versa. Lattice-mismatching (LMM) in adjacent crystal materials causes lattice strain, which, when high enough, is usually manifested in dislocations, fractures, wafer bowing, and other problems that degrade or destroy electrical characteristics and capabilities of the device. Unfortunately, the semiconductor materials that have the desired bandgaps for absorption and conversion of radiant energy in some energy or wavelength bands do not always lattice match other semiconductor materials with other desired bandgaps for absorption and conversion of radiant energy in other energy or wavelength bands.
Co-pending U.S. patent application, Ser. No. 10/515,243, addressed this problem for low bandgap (e.g., less than 1.35 eV), monolithic, multi-bandgap devices in order to convert lower energy, infrared radiation (e.g., 918 to 3483 nm) to electricity more efficiently. By the use of combination of cells lattice-matched (LM) to InP substrates, lattice constant transition layers and lattice-mismatched (LMM) cells, inverted monofacial and bifacial monolithic structures, ultra-thin monolithic, multi-band-gap, tandem structures, and other features, the inventions in that co-pending patent application could provide monolithic, multi-band-gap, tandem cells with band gaps in various combinations ranging from about 1.35 eV down to as low as about 0.36 eV. That range comprises invisible infrared light. There are also well-developed medium and high band gap, lattice-matched semiconductors grown on GaAs or Ge substrates, such as GaAs, GaInAsP, AlGaAs, GaInP, and AlGaInP. It was suggested in the co-pending U.S. patent application, Ser No. 10/515,243, that the low-bandgap, monolithic, multi-band-gap, tandem converters disclosed therein could be joined mechanically to higher band gap cell structures. However, prior to this invention, monolithic, multi-band-gap, tandem (MMT) solar photovoltaic (SPV) converters with lattice-matched subcells in the medium to high bandgap ranges have not included any subcells in the lower band gap ranges, such as lower than about 1.4 eV, with the exception of designs that incorporate lattice-matched Ge subcells. Therefore, prior to this invention, ultra-high-efficiency, monolithic, multi-band-gap, tandem, solar photovoltaic (SPV) converters have not been demonstrated due to the lack of a suitable, high-performance, optimum, low-band-gap subcell option to combine with the medium to high bandgap subcells in such structures.